Microwave frequency discriminators



y 1965 A. s. WILTSHIRE EI'AL 3,197,707

MICROWAVE FREQUENCY DISCRIMINATORS Filed Nov. 27. 1961 Y 2 Sheets-Sheet1 [g AMPLIEIER 32 MODULATOR PHASE SHIFTER 35 AMPLIFIER 29 7 TUNING \40HYBRID a MECl-MN/SM JUNCTION Pl-MSE SENSITIVE PHASE DETECT R SENSITIVEDETECTOR PHASE A SH/FTER 4 FIG 3 2 7 3 DETECTOR TTENUAT 1 HYBRID M'XER11 JUNCTION AMPLIFIER 4 PHASE SENSITIVE DETECTOR United States Patent3,197,707 MICROWAVE FREQUENCY DISCRIMINATORS Allan Stanley Wiltshire andSidney Albert Drage, both of Malvern, England, assignors to James Scott(Electronic Engineering)-Lirnited, Glasgow, Scotland Filed Nov. 27,1961, Ser. No. 155,136 Claims priority,application Great Britain, Nov.28, 1960, 40,79 8/ 60 10 Claims. (Cl. 325-487) This invention relates tomicrowave frequency dis criminators and particularly to suchdiscriminators of the bridge type.

Certain microwave structures, e.g. hybrid junctions, exhibit properties:comparable with a classical bridge circuit in that zero output may beobtained when certain elements. of the structure are balanced. Such amicrowave arrangement is referred to hereinafter as a microwave bridge.

One of the applications of a microwave frequency discriminator is in theexamination of unwanted noise modulations in the output of a continuouswave klystron oscillator.

Any practical oscillator has a power spectrum of finite proportions,i.e. the assumption frequently made for convenience is not true becausethe output power is in fact not confined to a particular frequency buthas a spectrum in which most of the output power is confined to a narrowband of frequencies in the region of maximum output with small butmeasurable power emitted over a range above and below the centerfrequency. This loworder power spectrum is produced by certainimperfections and instabilities in the source generator, its powersupplies and associated circuits and generally has the characteristicsof noise.

When the generated noise spectrum extends into the region of transmittedintelligence in a communications system, its presence will impose alimit on the maximum level at which intelligence may be transmitted,received and detected. It is therefore desirable to provide means fordetermining the noise power spectrum of the generator in regionscorresponding to the required bands of intelligence transmission and thepresent invention aims at providing such means.

Ifthe. communications system. is of therfrequeney modulated QFM) type itis desirable to provide means for determining. the FM component of thenoise spectrum, and similarly if the system is: of, the amplitudemodulated (AM) type. If the system is of the. narrow band, highresolution type, for example: a CW radar, their it is-necessarytea-determine the noise power spectrum with a resolution at least ashigh; as the resolution of the radar receiver for the: measured: resultstobe meaningful;

For convenience the performance criteria may be arbitrarilydefined as:

(1) Long-term variations in the frequency and amplitude of the outputfrom the oscillator, including cyclic variations having periods ofhours, minutes, seconds etc.;

(2) Medium-term variations with cyclic components having frequenciesranging from 1 or 2 cycles per second (c./s.)' to about 100. c./s.;

100,000 c./s., and

(4) Very short-term variations from 100,000 c./s. upwards.

Generally it is impractical to devise a systemto measure theinstabilities occurring in allthe above regions at the same time andwith equalsensiti'vity. In practice the alternative systems are designedto provide optimum performance in one or tworegions of specificinterest. Thus a microwave frequency discriminator of the type disclosedin U.S. Patent No. 2,883,533, with electromechanical switching of theoutput signals to the phasesensit-ive detector, would provide signalscapable of measuring and controlling long-term variations, but

would not respond to higher order variations of the order of severalhundred c./s'.

Discriminatorslike that disclosed in US. Patent No. 2,770,729, which bytheir mode of behavior generate noise extending into the wanted signalspectrum region at levels higher than the noise generated in. klystronoscillators under the best possible conditions, may be satisfactory forstabilizing the mean frequency output of a klystron relative to thesetting of a standard resonant cavity, but are unable, because of aninternal noise limitation, to transmit the low level, higher order noisepresent in the output from the control oscillator and therefore areunresponsive to noise fluctuations occurring in this region.

US. Patent No. 3,079,563 relates to a microwave frequency discriminatorhaving an input arm to which a has been found practicable to achievecarrier cancella tionratios offrom 30 db to SO- db with thisdiscriminator using manual tuning means for the cavity. Highly sensitivethough this discriminator is, the continuing. advances in microwavetechnique have led to a requirement to achieve carrier cancellationratios of a higher order so that the limiting sensitivity of the systemcould be correspondingly improved,

Such an increase in sensitivity could be attempted by increasing thepower of the input signal- However this suffers from the disadvantage ofincreasing the chance of' burn-out of crystals used in the signal.channel, and thefiicker noise in the crystals is also more likely to beincreased; Accordingly the present invention aims at providing amicrowave frequency discriminator including a resonant cavity which isautomatically tuned by a feed back loop so that the carrier is cancelleddespite longterm frequency variations.

According to a further preferred feature of the present invention asecond feed-back loop is provided to suppress medium-term variations inthe carrier signals.

One example of a discriminator. of the present inventiorr will now bedescribed with reference to the accompanying drawings in which:

FIG, 1 is a block diagram of the, basic circuit;

Patented July 27, 1965 Because FIG. 2 is the circuit of onediscriminator of the present invention, and

FIG. 3 is a diagram of a refinement of the circuit shown in FIG. 2.

A description of the basic structure of the discriminator will be givenfirst and followed by a description of added control loops according tothe invention which automatically control the balance of thediscriminator bridge.

As shown diagrammatically in FIG. 1, the signal of which the noisemodulation is to be measured passes from the source SW along an inputarm 7 to a hybrid junction 1 having a balancing arm 3 including avariable attenuator and a variable short circuit 4, an arm 8 leading toa cavity resonator 2, and an output arm 6 leading to a firstphase-sensitive detector 11 and a second detector 22.

Input reference signals in the arm 7 also pass through a directionalcoupler 12 and channel 13 to the detector 11 and through a 90 phaseshifter 23 to the detector 22.

One output from the detector 11 passes to means 26 for tuning the cavity2 A second output passes through an amplifier 42 to an adjustableattenuator 32 in one branch of a parallel attenuating circuit in theother branch of which is a 90 phase shifter 35 and an adjustableattenuator 31 connected through an amplifier 45 and channel 43 to theoutput of the second detector 22. The attenuators 31 and 32 arepreferably electrically adjustable.

Equal signals are passed into both halves of the parallel attentuatingcircuit from the input arm 7 through a directional coupler 29 having abalancing impedance 36.

The output of the parallel circuit passes through a directional coupler34 to the output arm 6, before the first detector 11.

By thi means the signal in the arm 6 has applied to it one component inphase and another component in quadrature with the carrier of the inputsignal, each of the components being controlled by one of the detectorsby means of the control amplifiers and electrically adjustableattenuators.

A preferred example of the present invention is illustrated in FIG. 2,in which parts indentical with those described in FIG. 1 have the samereferences.

In FIG. 2 the microwave bridge is formed by a hybrid coupling 1 (a threedb directional coupling) to which is connected a resonant cavity 2, abalancing arm 3 terminated by a variable short-circuit 4 and containinga variable attenuator 5, an output arm 6 and an input arm 7 arranged tobe fed from a source SW of which the noise modulation is desired to bemeasured. The output arm 6of the bridge is connected via a mixer 9 andan intermediate frequency (I.F.) signal amplifier 10 to a phasesensitive detector 11.

The bridge is balanced by adjustment of the variable attenuator 5 andthe adjustable short circuit 4; the zero output point of the bridgeoccurs when the source carrier frequency and cavity resonant frequencyare equal.

If a frequency deviation is present in the. feed from the source SW thenan output signal appears in the output arm 6 of the bridge;

A reference signal for the phase sensitive detector 11 is obtained fromthe source SW by means of a directional coupling 12 which is inserted inseries with the input arm 7 of the microwave bridge. The directionalcoupling 12 couples to' a reference channel 13 which contains anadjustable attenuator 19, and adjustable phase shifter 14,-

a mixer 15 and an LP. reference signal amplifier 16 of which the outputis fed as a reference signal to the phase sensitive detector 11. Themixers 9 and 15 are fed from a common local oscillator 17.

The output of the phase sensitive detector 11 varies in' amplitudecorrespondingly to the frequency deviation originally present in theinput signal to the bridge from the source SW.

The signal fed to the phase sensitive detector 11 is ina sensitive toamplitude modulations present in the original signal.

A sensitive indication of amplitude modulation is ob tained by a secondphase sensitive detector 22 provided as shown and fed from the LP.signal and reference amplifiers 1i) and 16 respectively, a phase-shifter23 being connected in series with the reference signal input. The signalthen appearing at the output 24 of the second detector 22 gives ameasure of the amplitude modulation present in the signal input to thebridge.

The sensitivity of the discriminatorthat is the sensitivity of FM to AMconve-rsion-can be varied by adjusting the power level input of thebridge from the source SW; the attenuator 19 in the reference channel 13can be calibrated and its setting necessary to maintain, for a givenbalance of the bridge, a pre-determined reference signal level at thephase sensitive detector 11, will be a measure of the change insensitivity.

A first control loop for automatic control of bridge balance is providedby a cavity adjustment mechanism 26, shown diagrammatically only, and amotor 27 which drives the mechanism 26 in response to frequency errorsignals appearing at the output (terminal 25) of the firstphase-sensitive detector 11; the frequency error signals are applied tothe motor 27 through an amplifier 28. The action of the first feedbackloop is to ensure that the cavity 2 is maintained at a resonantfrequency close to the mean frequency of the signal applied SW. Theeffects of this are two-fold, firstly cancellation of carrier componentin the signal arm 6 ismaintained and high power can therefore becontinuously applied to the input arm 7 of the bridge 1. Secondly, thesystem is maintained Within the operating region of the bridge wherelinear conversion of PM sidebands on the input arm '7 to AM sidebands onthe output arm 6 is maintained.

A second control loop is provided by a directional coupling 2.9 (10 db)connected through a phase-shifter 38 to two ferrite modulators 31, 32connected in parallel between two hybrid junctions 43, 49, and a furtherdirectional coupler 34 (eight db) feeding to the output arm 6 of thediscriminator bridge before the input to the signal mixer 9.

The modulator 31 is connected in series with a quadrature phase-shiftcircuit 35. Balance impedauces 36, 37, 38, 39 are also provided.

The modulator 31 is controlled by a signal from the second (amplitudemodulation) phase-sensitive detector 22 which is fed to it through achannel 43 and a lowpass filter 44 and an amplifier 45.

The modulator 32 is controlled by a signal from the output of the first(frequency modulation) phase-sensitive detector 11, the signal beingsupplied through channel .0, low-pass filter 41 and an amplifier 42.This second control loop is incorporated into the system to allowmeasurements to be made when the signal applied is frequency modulatedat low frequency but with high deviation. Such a signal, commonly occursin the system for which this measuring equipment is designed.

Without this cancellation loop, these low frequency signals would resultin the transfer of a considerable amount, of power into the arm 6 whichwould then be applied to the crystal m'mer 9 (shown in FIG. 2), whichwould thereby be damaged, or possibly burnt out. The cancellation loopgenerates an equal and opposite microwave signal in the coupler 34 toreduce this low frequency component to a safe level.

Thus, if we consider that the input SW consists of a microwave signalwith the undernoted characteristics, the discriminator described willsuccessfully measure the unwanted components without damage orinaccuracy.

In operation the tuning mechanism 26, which is conveniently amotor-driven cavity tuning plunger, is controlled by amplified frequencyerror signals from the out put of the phase-sensitive detector 11. Themotor 27 is biassed so that the motor is not energised when the error iswithin a chosen range, e.g. :40 kc./s. at an operating frequency of Kmc./s.

Equal components of the carrier of the input signal from the source SWare obtained from the input arm 7 by means of the directional coupler29- and the phaseshifter 30', the hybrid-junction 49-divid'ing the inputsignal from the coupling 29 into two equal parts. The phaseshift circuit35 ensures that the component. fed to the modulator 31' is in quadraturewith that fed to the modulator 32.

From the modulatorsiil 32 the. quadrature components are combi'nedin thehybrid junction 48 and, by means of the directional: coupler 34,.applied to the bridge output" before the input of the signal mixer 9.The amplitudes of the components are determined by the ferritemodulators 31, 32, under the control of the outputs of thephase-sensitive detectors 11 and 22. Thus, any small out-of-balancewhich exists, particularly when the cavity 2 is under the control of thetuning mechanism, is dealt with by these quadrature components of theinput signal carrier fed into the signal mixer 9.

In a typical discriminator operating at 10K mc./s. (3 cm./s. wavelength)the LF. (intermediate frequency) was chosen at 40 rnc./s. and the secondcontrol loop had a gain of 55 db at frequencies up to 1 c./ s. fallingoff beyond that frequency at 6 db per octave. This enables an overallbalance better than 70 db to be attained: the input carrier power was5W.

When starting, the discriminator may or may not be correctly phasedthroughout and the correct conditions are attained by adjustment of thephase-shifter 30, and the phase-shifter 14.

It may be noted that the discriminator is able to follow an FMtransmission and still measure any AM or FM noise present on thecarrier; this presupposes that the modulation frequency is well withinthe bandwidth of the second control loop.

Typical characteristics of the low-pass filters 41, 44 are:

Frequency, c./s Response, db

6. 16 }and thereafter --6 12 (lb/octave.

A feature which could usefully be added is illustrated in FIG. 3,showing how the amplitude of the microwave leakage signal carrier at themixer 9 can be limited automatically by inserting a ferrite attenuator50 in the input wavegmide 7 between the couplings 12 and 29 andcontrolling it by means of a signal from a detector 51 at the output ofthe IF amplifier 10.

An alternative arrangement has been devised for the second control loopin which two ferrite amplitude controllers themselves controlled by theoutputs of the phasesensitive detectors provide controlled mismatches,conveniently in the arm 3 of the bridge. Ferrite devices are known whichcan produce a voltage-standing-wave ratio in a waveguide of anymagnitude and phase. Typically two stub slots spaced M 8 apart andleading-off the walls of a main waveguide each contain a ferrite blockwithin the field of an energising coil. Variation of current in thecoils varies the magnitude and phase of a standing wave in the mainwaveguide. In the bridge the waveguide is connected across in serieswith the waveguide constituting arm 3 and a phase-adjuster is providedto assist in establishing correct phase relationships at starting up.

We claim:

1. A microwave frequency discriminator comprising a microwave bridge andtwo phase-sensitive detectors, the microwave bridge including an inputarm, an output arm, a resonant cavity, an adjustable impedance, an armconnected to the resonant cavity and an arm connected to the adjustableimpedance, a hybrid junction to which the arms are connected, the twophase-sensitive detectors each including two input arms and an outputarm and being connected in parallel. with each other, an output signalchannel connected between the bridge output arm and an input arm ofeachphase-sensitive detector, a reference signal channel connectedbetween: the bridge input arm and the other input arm ofi each of thephase-sensitive detectors, means connected between an input arm-of eaclrphase-sensitive detector to apply to the second phasesensitive detectora signal derived. from; and in quadrature with, that. applied to thefirst phase-sensitive detector, resonant cavity tuning, means, and.means to' control the; cavity tuning means, the control means. being,connected between the output arm of the-first phase-sensitive detectorand the cavity tuning means.

2. A discriminator as claimed in claim 1, in which the phase-sensitivedetectors operate at intermediate frequency, the bridge output arm andthe reference signal channel each including a mixer, the two mixersbeing fed by a common local oscillator.

3. A discriminator as claimed in claim 1, in which the reference signalchannel includes a variable attenuator and a variable phase shifter.

4. A discriminator as claimed in claim 1, in which means are provided toproduce controlled mismatches in one of the arms of the bridge, thedegree of the mismatches being determined by the outputs of the twodetectors.

5. A discriminator as claimed in claim 4, in which the said one armcomprises the balancing arm of the bridge, and the controlled mismatchproducing means comprises two ferrite amplitude controllers connected tothe output arms of the two phase-sensitive detectors.

6. A discriminator as claimed in claim 5, in which each amplitudecontroller is in the form of a ferrite block mounted within a stubwaveguide leading oh the main waveguide of the balancing arm, the blockbeing within the field of a coil energised by the respective detector,the stub waveguides being spaced apart along the length of the mainwaveguide.

7. A microwave frequency discriminator comprising a microwave bridge andtwo phase-sensitive detectors, the microwave bridge including an inputarm, an output arm, a resonant cavity, an adjustable impedance, an armconnected to the resonant cavity and an arm connected to the adjustableimpedance, a hybrid junction to which the arms are connected, the twophase-sensitive detectors each including two input arms and an outputarm and being connected in parallel with each other, an output signalchannel connected between the bridge output arm and an input arm of eachphase-sensitive detector, a reference signal channel connected betweenthe bridge input arm and the other input arm of each of thephase-sensitive detectors, means connected between an input arm of eachphase-sensitive detector to apply to the second phasesensitive detectora signal derived from, and in quadrature with, that applied to the firstphase-sensitive detector, resonant cavity tuning means, means to controlthe cavity tuning means, the control means being connected between theoutput arm of the first phase-sensitive detector and the cavity tuningmeans, means for introducing into the bridge output arm two signalcomponents, of which one is in phase with, and of which the other is inquadrature with, the carrier of the signal applied to the bridge inputarm, means to control the amplitude of one signal component, the meansbeing connected to the output arm of the first phase-sensitive detector,and means to control the amplitude of the second signal component, themeans being connected to the output arm of the second phasesensitivedetector.

8. A discriminator as claimed in claim 1, in which the resonant cavitytuning means comprises a tuning plunger within the cavity and thecontrol means therefor comprises a motor operatively connected to theplunger and an amplifier connected between the output arm of the firstphase-sensitive detector and the motor.

9. A discriminator as claimed in claim 1, including 3 means forintroducing into the bridge output arm two signal components, of whichone is in phase with, and of which the other is in quadrature With, thecarrier of the signal applied to the bridge input arm, a first modulatorto control the amplitude of one of the signal components under theinfluence of the first phase-sensitive detector, a second modulator tocontrol the amplitude of the other of the signal components under theinfluence of the second phase-sensitive detector, and means connectedbetween the modulators and the output arm of the bridge to feed themodulated signal components to the output arm of 8 the bridge before thefirst and second phase-sensitive detectors.

10. A discriminator as claimed in claim 9, in which a low pass filterand an amplifier are connected between 5 each of the phase-sensitivedetectors and its respective modulator.

References Cited by the Examiner UNITED STATES PATENTS 2,770,729 11/56Dicke 331-9 DAVID G. REDINBAUGH, Primary Examiner.

1. A MICROWAVE FREQUENCY DISCRIMINATOR COMPRISING A MICROWAVE BRIDGEINCLUDING AN INPUT ARM, AN OUTPUT ARM, MICROWAVE BRIDGE INCLUDING ANINPUT ARM, AN OUTPUT ARM, A RESONANT CAVITY, AN ADJUSTABLE IMPEDANCE, ANARM CONNECTED TO THE RESONANT CAVITY AND AN ARM CONNECTED TO THEADJUSTABLE IMPEDANCE, A HYBRID JUNCTION TO WHICH THE ARMS ARE CONNECTED,THE TWO PHASE-USENSITIVE DETECTORS EACH INCLUDING TWO INPUT ARMS AND ANOUTPUT ARM AND BEING CONNECTED IN PARALLEL WITH EACH OTHER, AN OUTPUTARM AND CHANNEL CONNECTED BETWEEN THE BRIDGE OUTPUT ARM AND AN INPUT ARMOF EACH PHASE-SENSITIVE DETECTOR, A REFERENCE SIGNAL CHANNEL CONNECTEDBETWEEN THE BRIDGE INPUT ARM AND THE OTHER INPUT ARM OF EACH OF THEPHASE-SENSITIVE DETECTORS, MEANS CONNECTED BETWEEN AN INPUT ARM OF EACHPHASE-SENSITIVE DETECTOR TO APPLY TO THE SECOND PHASESENSITIVE DETECTORA SIGNAL DERIVED FROM, AND IN QUADRATURE WITH, THAT APPLIED TO THE FIRSTPHASE-SENSITIVE DETECTOR, RESONANT CAVITY TUNING MEANS, AND MEANS TOCONTROL THE CAVITY TUNING MEANS, THE CONTROL MEANS BEING CONNECTEDBETWEEN THE OUTPUT ARM OF THE FIRST PHASE-SENSITIVE DETECTOR AND THECAVITY TUNING MEANS.